Vibrational investigation of the antiferrodistortive structural instabilities in the perovskite crystal RbCaF3

The Raman spectra of the three phases of the perovskite RbCaF3 have been recorded. A detailed group-theoretical analysis together with a rigid-ion model calculation in the cubic symmetry have been carried out in order to identify the observed peaks. The established compatibility relations demonstrate that the orthorhombic Gamma(0,0,0) modes come from the Gamma(0,0,0) and X(0.5,0.5,0) tetragonal modes, themselves coming from the Gamma(0,0,0), R(0.5,0.5,0.5), M(0.5,0,0.5), and X(0,0.5,0) eigenmodes of the cubic Brillouin zone. These calculations, together with the analysis of experimental Raman spectra, unambiguously show that the first-order transition that occurs at T-c = 31.5 +/- 1 K gives rise to an orthorhombic symmetry (Pnma space group associated with the a(-)a(+)a(-) tilt system in Glazer's classification) without any group-subgroup relation to the tetragonal one. Particular attention was also paid to hard Raman modes which are not directly involved in the transition but still provide valuable information about the local symmetry. The temperature dependence of the integrated Raman intensity of these hard modes is correctly described by a power law t(2 beta) in which t is the reduced temperature, (T-T-c)/T-c, and the critical exponent beta is found to be 0.27. A deviation from this behavior becomes apparent close to transition, indicating that local structural distortions subsist over a fairly wide temperature range.